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<a name="Running-a-function-on-all-layout-objects"></a>
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<h4 class="subsection">6.5.2 Running a function on all layout objects</h4>
<p><a name="index-calling-code-on-layout-objects-2829"></a><a name="index-g_t_005capplyOutput-2830"></a><a name="index-g_t_005capplyOutput-2831"></a>
<p>The most versatile way of tuning an object is <code>\applyOutput</code>. Its
syntax is
<pre class="example">\applyOutput <var>context</var> <var>proc</var>
</pre>
<p class="noindent">where <var>proc</var> is a Scheme function, taking three arguments.
<p>When interpreted, the function <var>proc</var> is called for every layout
object found in the context <var>context</var>, with the following
arguments:
<ul>
<li>the layout object itself,
<li>the context where the layout object was created, and
<li>the context where <code>\applyOutput</code> is processed.
</ul>
<p>In addition, the cause of the layout object, i.e., the music
expression or object that was responsible for creating it, is in the
object property <code>cause</code>. For example, for a note head, this is a
<a name="index-NoteHead-2832"></a><a href="../lilypond-internals/NoteHead.html#NoteHead">NoteHead</a> event, and for a <a name="index-Stem-2833"></a><a href="../lilypond-internals/Stem.html#Stem">Stem</a> object,
this is a <a name="index-NoteHead-2834"></a><a href="../lilypond-internals/NoteHead.html#NoteHead">NoteHead</a> object.
<p>Here is a function to use for <code>\applyOutput</code>; it blanks
note-heads on the center-line:
<pre class="example">(define (blanker grob grob-origin context)
(if (and (memq (ly:grob-property grob 'interfaces)
note-head-interface)
(eq? (ly:grob-property grob 'staff-position) 0))
(set! (ly:grob-property grob 'transparent) #t)))
</pre>
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